1. Field of the Invention
The present invention relates to the protection of users of electronic devices against potentially hazardous contact with high-voltage power supplies. More specifically, the present invention relates to a safe and inexpensive external A/C adapter with a high-voltage D/C output for use in small, low-cost power systems.
2. The Background
The dangers of electric shock from the careless use of common electronic appliances, from personal computers to musical christmas-tree lights, are well-known to those of ordinary skill in the art. A small current of only a few thousandths of an ampere across the heart of a human victim is enough to cause cardiac arrest and death in certain circumstances. Such electronic appliances and their associated power supplies have nevertheless become ubiquitous fixtures of modern life. The sheer familiarity of these devices, combined with the fact that power supplies are often quite small and innocuous in appearance, increase the likelihood that someone who is either unable to appreciate, or simply unaware of the threat--young children or infants, for example--will be exposed to dangerously high voltages, herein defined as any voltage exceeding 60V in absolute value relative to a ground reference.
For this reason, high-voltage power supplies are required to meet significant, and often costly, safety requirements well-known to those of ordinary skill in the art. One of these requirements is that any such safety system must be designed redundantly, so that no single failure is sufficient to compromise the safety system as a whole.
In consumer-electronics applications, the most common type of power supply is the external A/C adapter. These adapters are used in conjunction with electronic devices designed to operate under D/C power at specific voltages, and allow such electronic devices to be powered from standard A/C sources, like ordinary wall outlets. An external A/C adapter for use with such consumer-electronic devices usually comprises a small box unit, approximately six to twelve inches in length, with an A/C input designed to plug into a wall outlet or other source of A/C power (typically 110V A/C at 60 Hz or 220V A/C at 50 Hz), and a D/C output in the form of a cord terminating in an output plug having a ground connector and one or more powered connectors carrying D/C voltage. The connectors are often of a female type to prevent accidental shorting.
The electronic device being powered by the adapter has a corresponding mating socket which typically takes the form of a recessed area in the chassis of the device. Within the mating socket, a plurality of uninsulated metallic pins is disposed, each of which join, or mate, with a respective one of the female connectors of the D/C output plug when the plug is inserted into the device's mating socket.
Housed within the adapter unit is a conventional power supply (either linear or switched) which converts the A/C input from a standard A/C source to the desired D/C output voltage level(s). In addition, the adapter contains rectification and control circuitry that converts the potentially noisy, sinusoidally-varying A/C input into a clean, constant D/C output suitable for proper device operation. The D/C output is often coupled to the A/C input indirectly through inductive means such as a transformer. Keeping the D/C output and A/C input portions of the adapter separate enhances the safety of the device, and for this reason, it is important that any control circuitry used to communicate between the A/C and D/C portions of the adapter be implemented in a way that does not link them directly. For instance, optical, magnetic, or capacitive coupling may be used.
Since the A/C adapter is external to the device, it is possible for the D/C output plug to be energized when the output plug is not yet safely seated in the device mating socket, leaving the "hot" D/C output plug dangerously free and exposed. Normally, some mechanical means of protection, in the form of specially designed connectors with deeply recessed outputs, are used to block physical access to the high-voltage D/C output of the adapter. Such special connectors, however, are typically large, cumbersome, and expensive.
Moreover, such prior-art mechanical precautions are often deficient in another respect: because the D/C output plug remains "hot" regardless of whether or not it is inserted in the device input socket, a danger exists of electric arcing and sparking between the D/C output plug and the device mating socket whenever the plug is inserted or removed. This phenomenon not only jeopardizes user safety; it can also destroy the delicate circuitry of the electronic device that draws power from the adapter.
Accordingly, it is an object and advantage of the present invention to provide a safe external A/C adapter that protects the user from exposure to dangerously high voltages without the need for a special mechanical connector, using simple internal electronic control circuitry.
It is another object and advantage of the present invention to provide a safe external A/C adapter with a D/C output that turns ON only after the D/C output plug has been safely inserted into the device input socket.
Yet another object and advantage of the present invention is to provide a safe external A/C adapter with a D/C output plug that turns OFF prior to being completely removed from the device input socket.
Yet another object and advantage of the present invention is to provide a safe external A/C adapter with independent backup circuitry so that a single failure in the system will not compromise the user's safety.
These and many other objects and advantages of the present invention will become apparent to those of ordinary skill in the art from a consideration of the drawings and ensuing description of the invention.